• Preheat oven to 350° F and line a baking sheet with parchment paper.
• In a medium bowl, cream room temperature butter and sugar until creamy.
• Beat in egg yolk. You can bring your egg to room temperature by placing in a bowl of warm water for 5 minutes.
• In another bowl, whisk flour, baking powder and salt.
• Mix milk and vanilla in a small cup.
• Alternate your wet and dry ingredients, mixing well after each addition, until your ingredients are fully blended.
• Divide dough in half then take approximately 2 tablespoons of dough from one half and add it to the other.

For the Chocolate Dough

• Mix your 3 tablespoons of cocoa powder and 1 tablespoon of oil in a small bowl. (If you have the unsweetened chocolate square you can melt one square instead which is the equivalent.) Since I don't bake often with unsweetened cocoa, I find it easier and more cost effective to keep cocoa powder such as Hershey's on hand as it has a longer shelf life then the squares. If you do buy the squares, I find they store longer in a drawer in the refrigerator.
• Blend the unsweetened chocolate with the half of vanilla dough that is minus the 2 tablespoons. Once you add the unsweetened chocolate you will have equaled it out again.
• Cover tightly with plastic wrap and chill dough for 2 hours or overnight.

Creating the Checkboard Pattern

• Remove dough from refrigerator. Makes sure your edges are all flat by pressing them against the cutting board before you take the plastic wrap off.
• Slice each dough block lengthwise into equal pieces.
• Cut each slice into strips.
• Assemble the strips, alternating vanilla and chocolate. Make as many columns and rows as you want.
• Wrap dough blocks and chill 2 hours or overnight again.
• Remove dough from refrigerator and slice into 1/4 inch cookies. Rotate dough every slice to keep the knife from pulling the dough out of shape.
• Line a baking sheet with parchment paper and evenly space the cookies. If you are still not happy with the shape, you have a second chance to reconfigure it once you place it on your cookie sheet. Just apply slight pressure on opposites sides. They will make 12-15 cookies depending on the thickness of your roll. These cookies should not spread so you can use a half size baking sheet.
• Bake for 12-15 minutes. Let cool slightly before you remove them so they don't break. Store in a covered container on the counter.